Let's see: A custom wall and a custom floor:
The custom floor should be relatively easy:
-Select the texture that you want to be used
-Create the specular and normal map (if you don't have a single clue about how to make them you can ask me to make them)
-Open the material editor, and tick the options: Diffuse, Specular, Normal. Also, the type of material should be "Soliddiffuse".
-In the "Diffuse" box, enter your texture, in the "Normal" your normal map (it doesn't mean a normal image, but the "Normal Map". More explanation here)
http://en.wikipedia.org/wiki/Normal_mapping
-In the specular, the specular map:
http://en.wikipedia.org/wiki/Specularity
After this, the material file shall be saved with the name of the diffuse, and in the folder where all textures are:
Textures->My textures->
Texture.mat
Texture.dds (or .png, .jpg) -----> The .dds are used for performance, but these work too
Texture_nrm.dds ---> Normal map
Texture_spec.dds ---> Specular map
This is the floor. Now we go to the wall, not as simple as seems.
Ok, ok: When we talk about a 3D object (like a wall) things get complicated. Way more than you could think.
As Harry Potter said, you need a modelling program (Blender/Maya/3DS Max). There are plenty of tutorials out there to help you with getting familiar with the programs.
Take in account that you'll still have to follow the steps of the floor (creating normal and specular map, creating the material, etc.)
If you need a wall, the best would be to do it the size of Amnesia's. You can ask me to do the wall if needed.